Abstract:

[Problems] To realize a log likelihood ratio calculation performed at a
higher speed while the circuit size and the power consumption are
reduced, regardless of the multilevel number of a modulation method.
[Means for Solving the Problems] A hard-decision bit of the bits
indicating the P-axial coordinate of a reception signal point is input to
an area detection circuit, and based on the hard-decision bit input, the
area detection circuit detects and outputs an area on the phase plane
where the coordinate of the reception signal point is present. A
soft-decision bit of the bits indicating the coordinate of the reception
signal point is input to an LLR circuit, and based on the soft-decision
bit input, the LLR circuit calculates a primary LLR. An LLR converter
calculates the final LLR based on an output (area detection result) from
the area detection circuit. With this configuration, a log likelihood
ratio is calculated while limiting the scope, within which the value of
the log likelihood ratio varies corresponding to the position of the
reception signal point, to a range between adjacent signal points
including the hard-decision threshold of the bit.

Claims:

1.-16. (canceled)

17. A log likelihood ratio arithmetic circuit for calculating a log
likelihood ratio from information of a coordinate of a reception signal
point to be applied to a communication system using a quadrature
amplitude modulation method, wherein the circuit limits a scope within
which a value of the log likelihood ratio varies corresponding to a
position of the reception signal point to a range between adjacent signal
points including a hard-decision threshold of a bit.

18. The log likelihood ratio arithmetic circuit, according to claim 17,
wherein the circuit sets a correspondence relationship between a bit and
a symbol such that a bit error rate becomes minimum.

19. The log likelihood ratio arithmetic circuit, according to claim 17,
wherein the circuit sets a correspondence relationship between a bit and
a symbol when the modulation method has signal points of an even power of
two in a Gray mapping.

20. The log likelihood ratio arithmetic circuit, according to claim 17,
wherein the circuit sets a correspondence relationship between a bit and
a symbol when the modulation method has signal points of an odd power of
two in a quasi-Gray mapping in which a bit error rate becomes minimum.

21. A log likelihood ratio arithmetic circuit, wherein the circuit
performs all or almost all arithmetic operation for calculating a log
likelihood ratio by a logic operation using information of a coordinate
of a reception signal point.

22. The log likelihood ratio arithmetic circuit, according to claim 18,
wherein the circuit outputs a soft-decision bit of 1CH included in the
information of the coordinate of the reception signal point expressed in
2CH directly or in an inverted state, as a log likelihood ratio of an
area where the log likelihood ratio varies.

23. The log likelihood ratio arithmetic circuit, according to claim 18,
wherein the circuit outputs a maximum value or a minimum value of the log
likelihood ratio, as a log likelihood ratio of an area where the log
likelihood ratio does not vary.

24. The log likelihood ratio arithmetic circuit, according to claim 18,
whereinwhen there is an area where it is impossible to perform area
decision based only on information of a hard-decision bit of 1CH included
in the information of the coordinates of the reception signal point
expressed in 2CH, the circuit outputs a maximum value or a minimum value
of the log likelihood ratio in an area where no bit error is caused, and
a soft-decision bit of CH having lower likelihood in 2CH in an area where
a bit error rate may be caused, under a condition in which the circuit
limits the scope within which the value of the log likelihood ratio
varies corresponding to the position of the reception signal point to the
range between adjacent signal points including the hard-decision
threshold of the bit.

25. The log likelihood ratio arithmetic circuit, according to claim 22,
wherein selection of the log likelihood ratio uses a result of area
decision performed based on the information of the hard-decision bit of
1CH included in the information of the coordinate of the reception signal
point expressed in 2CH.

26. The log likelihood ratio arithmetic circuit, according to claim 24,
wherein selection of the log likelihood ratio uses a result of area
decision by referring to a low-order bit of the hard-decision when there
is an area where it is impossible to perform area decision based only on
the information of the hard-decision bit of 1 CH included in the
information of the coordinate of the reception signal point expressed in
2CH.

27. The log likelihood ratio arithmetic circuit, according to claim 22,
wherein the log likelihood ratio is converted from a soft-decision bit
into another value.

28. A transmission apparatus comprising a log likelihood ratio arithmetic
circuit for calculating a log likelihood ratio from information of a
coordinate of a reception signal point to be applied to a communication
system using a quadrature amplitude modulation method, wherein the log
likelihood ratio arithmetic circuit limits a scope within which a value
of the log likelihood ratio varies corresponding to a position of the
reception signal point to a range between adjacent signal points
including a hard-decision threshold value of a bit.

29. A log likelihood ratio arithmetic method for calculating a log
likelihood ratio from information of a coordinate of a reception signal
point to be applied to a communication system using a quadrature
amplitude modulation method, wherein a scope within which a value of the
log likelihood ratio varies corresponding to a position of the reception
signal point is limited to a range between adjacent signal points
including a hard-decision threshold value of a bit.

[0002]In recent years, as demands for higher gain to error-correcting
codes to be applied to communication systems have increased, higher-gain
codes of a type of performing iterative decoding using soft-decision
codes such as turbo (convolution) code, LDPC code, and turbo product code
are increasingly used. Further, as the quantity of communications
increases regardless of transmission systems such as wireless/wired
communication systems or storage systems such as recording mediums,
demands for larger-capacity communication systems have also increased.

[0003]Input signals of a decoder used for such high-gain codes are signals
called LLR. Originally, an LLR of a quadrature modulation (quadrature
amplitude modulation: QAM) in which signal point is arranged
two-dimensionally is calculated based on a square distance of the
reception signal points expressed in two dimensions and all signal points
(16 pieces if 16 QAM). Since the square distance calculation is
complicated, there has been used a method generally in which a table
computed and generated beforehand by a program is stored in a ROM (Read
Only Memory) or in a logic circuit (truth table) corresponding to a ROM,
and an LLR is calculated with reference to the stored table (see, for
example, Non-Patent Document 1).

[0004]Further, as an apparatus of calculating an LLR by arithmetic
operation, Patent Document 1 discloses an apparatus which calculates a
log likelihood ratio of each bit in an M-ary OAM modulated signals to be
transmitted in a communication system efficiently in order to reduce the
time arithmetic time of LLR.

[0007]In the case of using an LLR calculation method described in
Non-Patent Document 1 in a system requiring related LLR, a reference
table which has been calculated and created is stored in a circuit such
as a ROM beforehand, and a signal including reception signal point
coordinates is input to the address of the circuit, whereby an LLR is
output.

[0008]If the method of referring to the table as described above is
applied to a system using a quadrature multilevel modulation (quadrature
amplitude modulation: QAM), the number of address bits of a ROM and the
number of output bits of a ROM become extremely large, which involves
difficulties in mounting due to the circuit size and the delay time.

[0009]Specifically, a reception signal coordinate signal of 2ch, in which
quadrature modulation is demodulated, is input to an ROM storing the
table described above, and the ROM outputs an LLR. In the case of a
multilevel modulation method, a reception signal is expressed by using a
plurality of bits. In such a case, as LLRs with respect to a plurality of
bits allocated to one symbol are required to be output simultaneously,
the number of output bits of the ROM is also large.

[0010]For instance, in the case of 128 QAM, if one soft-decision part of a
quadrature channel has 3 bits, one reception signal point is expressed in
(4+3)*2=14 bits, in which 4 bits of the hard-decision part are also
included. If an LLR for 1 bit includes 5 bits, the LLRs for 7 bits to be
transmitted in one symbol have 35 bits in total. In order to convert the
reception signal point of this case into LLRs, a ROM for 14-bit-input and
35-bit-output is required to be prepared, and to realize such a ROM, the
circuit size becomes extremely large. Further, as a delay time from an
address input to a data output is long, a high-speed operation cannot be
performed.

[0011]Further, in the apparatus for performing arithmetic operation
described in Patent Document 1, the amount of operation is reduced by
using symmetrical property in Gray mapping (correspondence between bits
and symbol). Therefore, this cannot be applied to a modulation method in
which the number of symbols is an odd power of two where the premise
conditions of Gray mapping are not satisfied. Further, as the final LLR
is calculated by square distance computation, the amount of arithmetic
operation to calculate the LLR is still large.

[0012]It is an object of the present invention to provide a log likelihood
ratio arithmetic circuit, a transmission apparatus, and a log likelihood
ratio arithmetic method, capable of calculating a log likelihood ratio at
a high speed while reducing the circuit size and the power consumption,
regardless of modulation method and bit accuracy of an LLR.

Means for Solving the Problems

[0013]A log likelihood ratio arithmetic circuit according to the present
invention is a log likelihood ratio arithmetic circuit for calculating a
log likelihood ratio from information of the coordinates of a reception
signal point to be applied to a communication system using a quadrature
amplitude modulation method, characterized in that the scope within which
the value of the log likelihood ratio varies corresponding to the
position of the reception signal point is limited to a range between
adjacent signal points including a hard-decision threshold value of the
bits.

[0014]A transmission apparatus according to the present invention is a
transmission apparatus including a log likelihood ratio arithmetic
circuit for calculating a log likelihood ratio from information of the
coordinates of a reception signal point to be applied to a communication
system using a quadrature amplitude modulation method. The log likelihood
ratio arithmetic circuit is characterized in that the scope within which
the value of the log likelihood ratio varies corresponding to the
position of the reception signal point is limited to a range between
adjacent signal points including a hard-decision threshold value of the
bits.

[0015]A log likelihood ratio arithmetic method according to the present
invention is a log likelihood ratio arithmetic method for calculating a
log likelihood ratio from information of the coordinates of a reception
signal point to be applied to a communication system using a quadrature
amplitude modulation method. The method is characterized in that the
scope within which the value of the log likelihood ratio varies
corresponding to the position of the reception signal point is limited to
a range between adjacent signal points including a hard-decision
threshold value of the bits.

EFFECTS OF THE INVENTION

[0016]According to the present invention, in a communication system using
a quadrature amplitude modulation method, calculation of a log likelihood
ratio, which is originally performed in two dimensions, is dissolved into
one dimension, and the amount of arithmetic operation can be reduced
significantly. Further, as all or almost all of the operation of log
likelihood ratio can be performed by a logical arithmetic unit, there is
no need to use a large-scale ROM, and the circuit size and the power
consumption can be reduced regardless of the modulation method and the
bit accuracy of the log likelihood ratio. Further, as the operating speed
of the circuit can be improved, a large-capacity communication system can
be realized. Accordingly, it is possible to increase the speed of
calculating a log likelihood ratio while reducing the circuit size.

[0017]Further, with a configuration to perform quasi-gray encoding using a
quadrature amplitude modulation method in which the bit error rate
becomes the minimum, it is also possible to perform LLR calculation at a
higher speed while reducing the circuit size even in the case of using a
quadrature amplitude modulation method having the signal points of an odd
power of two.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018]An exemplary embodiment of the invention will be described below
with reference to the drawings.

[0019]In a communication system performing modulation and demodulation, a
reception signal point coordinates which is an output of a demodulator
are expressed as a combination of orthogonal signals of two channels. In
the bits (m+n) of the signal, the upper m bits indicate a hard-decision
signal (bits capable of specifying a modulated symbol), and the lower n
bits indicate a soft-decision signal (bits indicating intermediate
position of the symbol).

[0020]As shown in FIGS. 1 and 13, a log likelihood ratio arithmetic
circuit according to the exemplary embodiment of the invention includes,
as the basic configuration, first arithmetic units (101A, 102A, 103A,
104C, 104D and 210) which are log likelihood ratio arithmetic circuits
for calculating a log likelihood ratio of a reception signal expressed in
two dimension and is capable of being dissolved into one-dimensional
signals, the circuits receiving one one-dimensional signals which is
input thereto and performing logical arithmetic operation using
information of the signal point coordinates of the one-dimensional signal
input, and second computation units (101B, 102B, 103B, 104D, 104E and
201) which receive the other one-dimensional signal input thereto and
performs logical arithmetic operation using information of the signal
point coordinates of the one-dimensional signal input to thereby
calculate a log likelihood ratio.

[0021]The first calculation units and the second calculation units limit
the scope in which the value of log likelihood ratio varies corresponding
to the position of the reception signal point to a range between adjacent
signal points including hard-decision threshold of a bit, and calculate
the final log likelihood ratio.

[0022]According to the exemplary embodiment of the present invention,
calculation of a log likelihood ratio performed two-dimensionally is
dissolved into one dimension, and the amount of computation can be
reduced significantly. Further, as all or almost all calculation of log
likelihood ratio can be configured by a logical arithmetic unit, there is
no need to use a large-scale ROM, and the circuit size and the power
consumption can be reduced, regardless of the modulation method and the
bit accuracy of the log likelihood ratio. Further, as the operating speed
of the circuit can be improved, a large-capacity communication system can
be realized. As such, a log likelihood ratio can be calculated at a
higher speed, while reducing the circuit size.

[0023]Further, with a configuration to perform quasi-gray encoding such
that the bit error rate becomes the minimum, it is possible to make the
circuit size smaller and to perform calculation of LLR at a higher speed
even in the case of using a quadrature amplitude modulation method having
signal points of an odd power of two.

First Exemplary Embodiment

[0024]Next, a log likelihood ratio arithmetic circuit which is optimum to
a quadrature amplitude modulation method having a signal point of an odd
power of two will be described as a first exemplary embodiment. FIG. 1 is
a block diagram showing an exemplary configuration of a log likelihood
ratio arithmetic circuit (LLR arithmetic circuit) according to the first
exemplary embodiment of the invention. In the present embodiment, an LLR
arithmetic circuit of 16 QAM will be described as an example. Further, in
the exemplary embodiment, it is assumed that the number of hard-decision
bits of 16 QAM is 2. Although the number of soft-decision bits varies
depending on the required characteristics, the number of soft-decision
bits is assumed to be 3 bits in the exemplary embodiment. Further, the
LLR arithmetic circuit shown in FIG. 1 is provided to a transmission
apparatus of a microwave wireless communication system using QAM
(quadrature amplitude modulation method) for example.

[0026]Note that if the area detection circuits 101A and 101B are
collectively indicated or either of them is indicated, they may be simply
indicated as an area detection circuit 101. Similarly, if the LLR
circuits 102A and 102B are collectively indicated or either of them is
indicated, they may be simply indicated as an LLR circuit 102. Similarly,
if the LLR converters 103A and 103B are collectively indicated or either
of them is indicated, they may be simply indicated as an LLR converter
103. Similarly, if the LLR converters 104A and 104B are collectively
indicated or either of them is indicated, they may be simply indicated as
an LLR converter 104.

[0027]To the area detection circuit 101A, a hard-decision bit among bits
indicating P-axis coordinates of a reception signal point is input, and
based on the hard-decision bit input, the area detection circuit 101A
detects and outputs an area on the phase plane where the P-axis
coordinates of the reception signal point are present.

[0028]To the area detection circuit 101B, a hard-decision bit among bits
indicating the Q-axis coordinates of the reception signal point is input,
and based on the hard-decision bit input, the area detection circuit 101B
detects and outputs an area on the phase plane where the Q-axis
coordinates of the reception signal point are present.

[0029]To the LLR circuit 102A, a soft-decision bit among bits indicating
the P-axis coordinates of the reception signal point is input, and based
on the soft-decision bit input, the LLR circuit 102A calculates a primary
log likelihood ratio (LLR).

[0030]To the LLR circuit 102B, a soft-decision bit among bits indicating
the Q-axis coordinates of the reception signal point is input, and based
on the soft-decision bit input, the LLR circuit 102B calculates a primary
log likelihood ratio (LLR).

[0031]In the exemplary embodiment, an output signal of the LLR circuit 102
is a primary LLR value only depending on a soft-decision bit. In the
exemplary embodiment, a primary LLR value output from the LLR circuit 102
is converted with a hard-decision bit, whereby the final LLR value is
calculated.

[0032]To the LLR converters 103 and 104, an output signal (primary LLR) of
the LLR circuit 102 is input, and based on the output (area detection
result) of the area detection circuit 101, the LLR converters 103 and 104
calculate the final LLR.

[0033]In the exemplary embodiment, an output signal from the LLR circuit
102 is applied with any one of the four conversions, including (1)
directly outputting the output signal from the LLR circuit 102, (2)
inverting and outputting the output signal from the LLR circuit 102, (3)
replacing the output signal with the predetermined maximum LLR value and
outputting it, and (4) replacing the output signal with the predetermined
minimum LLR value and outputting it, according to the detection result of
the area detection circuit 101. Then, the signal is output as the final
LLR value.

[0034]FIG. 2 is a block diagram showing an exemplary circuit configuration
of the LLR converters 103 and 104. As shown in FIG. 2, each of the LLR
converters 103 and 104 include a selection circuit 110 and an inverter
111.

[0035]The inverter 111 inverts the bit of the output signal of the LLR
circuit 102 and outputs.

[0036]To the selection circuit 110, an output signal (primary LLR) from
the LLR circuit 102, an output signal from the LLR circuit 102 which is
inverted by the inverter 111, the predetermined maximum LLR value, and
the predetermined minimum LLR value are input. Further, according to the
detection result of the area detection circuit 101, the selection circuit
110 selects any one of the output signal from the LLR circuit 102, the
output signal inverted by the inverter 111, the predetermined maximum LLR
value, and the predetermined minimum LLR value, and outputs it. Note that
the predetermined maximum LLR value and the predetermined minimum LLR
value are stored in a storage device such as a memory for example
beforehand.

[0037]The area detection circuit 101 determines, from the position of the
reception signal point, where the reception signal point are present
among the four areas including (1) an area in which an LLR varies with a
positive inclination crossing the threshold of hard decision, (2) an area
in which an LLR varies with a negative inclination crossing the threshold
of hard decision, (3) an area in which an LLR does not vary at the
maximum value without crossing the threshold of hard decision, and (4) an
area in which LLR does not vary at the minimum value without crossing the
threshold of hard decision. Then, the area detection circuit 101 outputs
a signal corresponding to the determination result.

[0038]Note that the processing performed by the area detection circuit
101, the LLR circuit 102, the LLR converters 103 and 104 is completely
same in the case of processing bits indicating the P-axis coordinates and
in the case of processing bits indicating the Q-axis coordinates.

[0039]In the exemplary embodiment, to the LLR arithmetic circuit, a
reception signal which is expressed in two dimensions and can be
dissolved into one-dimensional signals (P-axis signal and Q-axis signal)
is input. In the LLR arithmetic circuit, a one-dimensional signal (P-axis
signal) which is derived by dissolving the reception signal is input to
the area detection circuit 101A, the LLR circuit 102A and the LLR
converters 103A and 104A which configure a first arithmetic unit for
calculating a log likelihood ratio (LLR) by carrying out logical
arithmetic operation using information of the signal point coordinates of
the one-dimensional signal input. Further, the other one-dimensional
signal (Q-axis signal) which is a one-dimensional signal derived by
dissolving the reception signals is input to the area detection circuit
101B, the LLR circuit 102B, and the LLR converters 103B and 104B which
configure a second arithmetic unit for calculates a log likelihood ratio
(LLR) by carrying out logical arithmetic operation using information of
the signal point coordinates of the input one-dimensional signal. With
these configurations, the LLR arithmetic circuit carries out all or
almost all of the arithmetic operation for calculating the log likelihood
ratio by performing logical arithmetic operation using information of the
reception signal point.

[0040]Next, operation will be described. Before describing specific
operation of the LLR arithmetic circuit, a method of calculating a log
likelihood ratio (LLR) will be described first.

[0041]An LLR in the multilevel quadrature modulation (QAM) method is
calculated for each of the bits allocated to a modulation symbol. For
example, in the case of 16 QAM, 16 pieces of modulation symbols are
included, and 4 bits are allocated to each symbol. As such, in the case
of using 16 QAM, LLRs for 4 bits are to be calculated from one reception
signal.

(LLR Definition)

[0042]A method of calculating an LLR will be shown below.

[0043]First, a method of calculating an LLR in the case of BPSK (binary
phase shift keying) which is one-dimensional modulation will be described
first. If the transmission signal is 2 values of ±1 and this is
one-dimensional modulation, LLR(λ) is defined by the following
equation (equation (1)):

[0044]In the equation (1), P(xi=b|yi) is a posterior probability where the
transmission signal when the reception signal yi is received is b, and i
is a suffix indicating the time.

[0045]Further, the posterior probability P(xi=b|yi) with respect to the
heat noise in which the amplitude becomes normal distribution is
expressed by using the equation (2), where the noise power is δ2.

[0046]Therefore, LLR(λ) is calculated by assigning the posterior
probability shown in the equation (2) to the equation (1) as shown in the
equation (3):

λ i = 2 yi σ 2 equation
( 3 ) ##EQU00003##

[0047]In the equation (3), assuming that 52 is a constant value and
proportional constant is disregarded including 62, LLR(Δi) becomes
the reception signal (yi) itself. Note that if a min-sum algorism is used
as a decoding algorism, setting δ2 to be constant will never affect
the decoding characteristics. Further, even in the case of using an
algorism other than the min-sum algorism, as the correction ability of
the error-correcting code which is the current object is high, only a
slight improvement in the signal-to-noise power ratio will make the error
rate outside of the measurable range. Consequently, it is reasonable to
understand that δ2 is constant.

[0048]FIG. 3 shows a positional relationship between the LLR and a
reception signal point in the BPSK. It is assumed that the LLR is
expressed in a complement number of 2, as a form suitable for operation
by the decoder in the later stage of the LLR arithmetic circuit. In this
case, as the final output of the demodulator, if the LLR after
demodulation is 0 or more, the reception bit is determined as 0, and if
the LLR is a negative value, the reception bit is determined as 1. In
other words, MSB of the LLR becomes the determination result. Note that
handling the LLR as a complement of 2 is not essential. In the case of
considering the LLR as an offset binary form, the polarity will be
opposite.

[0049]Next, a method of calculating the LLR in the case of using a
one-dimensional modulation method described above is extended to
quadrature modulation (QAM, or two-dimensional modulation method). Even
in the case of using multilevel QAM in which a plurality of bits
correspond to one symbol, one bit is expressed in two values of ±1. If
it is 16 QAM, effects by all 16 pieces of signal points will be
calculated for 4 bits by using the equation (2), respectively. Note that
yi in the equation (2) is expressed by using quadrature coordinates, and
square distance calculation within exp of the equation (2) is performed
two-dimensionally.

[0050]Although the computation of the LLR described above does not require
any restriction for mapping, it is impossible to calculate with a simple
form as shown in the equation (3). Therefore, it is extremely difficult
to perform the calculation by the arithmetic circuit constructed as
hardware. As such, it is necessary to calculate the LLR for all reception
signal points using a calculator beforehand, and to save in the ROM in
the table form.

[0051]Further, as the modulation multilevel number is larger, the number
of bits indicating a reception signal point and the number of bits
allocated to one symbol become larger. Therefore, in order to output all
LLRs for each symbol with the reception signal point coordinates being an
input, the size of ROM for storing the LLR becomes extremely large, which
cannot be realized practically.

[0052]As an example of a circuit using two-dimensional modulation method,
a document "Product Specification AHA4541" (hereinafter Document A)
describes that AHA4541 which is an LSI for turbo product code made by the
US AHA Corp. can support up to 256 QAM. However, Document A fails to
disclose an LLR calculation method used for the LSI (AHA4541).

[0053]Further, "AHA Application Note Non-Square QAM Implementation for
aha4540" (Non-Patent Document 1) which is a document describing the
AHA4541 which is an LSI for turbo product code made by the US AHA Corp.
discloses, in Section 4.1, a program for calculating an LLR. However,
Non-Patent Document 1 nowhere discloses an operating method capable of
being performed by hardware and the circuit configuration.

[0054]Further, as a method of realizing LLR calculation with a simple
operation, there is one which is disclosed in Japanese Patent Laid-Open
Publication No. 2002-330188, "Method and Apparatus for Bit Log Likelihood
Ratio Operation of QAM Signal" (Patent Document 1).

[0055]The art disclosed in Patent Document 1 strictly calculates an LLR
based on an LLR definition formula by DSP (digital signal processing
LSI), and discloses a method of simplifying the LLR calculation. On the
other hand, the method disclosed in Patent Document 1 can significantly
reduce the originally required number of operations by gray-coding the
map and utilizing the symmetrical property.

[0056]However, mapping by gray coding cannot be realized in a modulation
method having the number of signal points of an odd power of two like 128
QAM. Regarding this aspect, Patent Document 1 indicates that the
application range is limited to the case of using a modulation method
which allows gray-coding (Carnot mapping). As such, even in the method of
Patent Document 1, an LLR is calculated as a difference of a square
distance between the position of a reception signal point and a signal
point corresponding to "0" and a signal point corresponding to "1", so
the operation is still complicated.

[0057]At least, an LLR arithmetic circuit which can be realized on
hardware supporting the QAM modulation method, in which the symbol number
is the odd power of two, is not present. In addition, an LLR arithmetic
circuit not requiring a squire distance calculation in a QAM modulation
method of the even power of two, having the same configuration of that of
the method of the odd power of two, is not well known.

[0058]In contrast, the exemplary embodiment can support the QAM modulation
method in which the number of symbols is the odd power of two, and
realizes an LLR arithmetic circuit which can achieve a simplified circuit
configuration even for the QAM modulation method in which the number of
symbols is the even power of two.

(Simplification when the Number of Symbols is Even Power of Two)

[0059]Next, simplification of a circuit configuration of an LLR arithmetic
circuit when the number of symbols of a reception signal is the even
power of two will be described. In order to realize LLR operation in
which the number of symbols is the even power of two on a circuit of a
practical size, it is necessary to use an arithmetic circuit based not on
a table format but on a predetermined rule. Hereinafter, a method of
significantly simplifying the LLR operation without deteriorating the
accuracy will be described.

[0060]First, for simplifying the operation, the following two assumptions
are given.

[0061]Assumption (a): bit error may be caused, and an area which is the
subject of LLR operation is limited to the part between two signal points
corresponding to different bits; and

[0062]Assumption (b): mapping is realized such that even a two-dimensional
signal can be independently calculated by each of the one-dimensional
signals.

[0063]Meaning of these two assumptions and the limitations thereof will be
described below.

[0064]The assumption (a) means that if the distance between signal points
is 2 d, the noise amplitude is 2 d or lower. For example, FIG. 4 shows an
example of positional relationship between an LLR and reception signal
point in 16 QAM. FIG. 4A indicates an exemplary positional relationship
between an LLR and a reception signal point in the case of Pch MSB, and
FIG. 4B indicates an exemplary positional relationship between an LLR and
a reception signal point in the case of Pch LSB.

[0065]As shown in FIG. 4A, in the case of Pch MSB, there are four signal
points P0 to P3. As shown in FIG. 4A, in the case of Pch MSB, the bit
value changes from 0 to 1 between the signal points P1 to P2. In this
case, if a reception signal point is present between P1 and P2, the
transmission signal point may be P1 or P2, and as the reception signal
point is closer to P1, it is determined that the possibility of the
transmission signal point being P1 is high, and that the transmission
signal point is never be P0 or P3.

[0066]In the example shown in FIG. 4A, a bit error is caused when the
noise amplitude exceeds d. If the possibility of causing a bit error is
about 0.01 or lower, the possibility that the amplitude of noise
according to normal distribution exceeds 2 d is about 1*10-6. In the
case where it is necessary to consider effects of noise having amplitude
of 2 d or more, an extremely large number of errors have been caused
between noises with amplitude of d to 2 d, and there is no gain in error
correction. In that case, no improvement is provided in the error rate by
error correcting operation.

[0067]Further, even calculation in which effects of all signal points are
considered is performed based on the definition of LLR, effects by signal
points other than the nearest signal points are extremely small, so that
the effects will not be reflected on the LLR value which is quantized to
about 3 to 5 bits. Accordingly, the assumption (a) will not deteriorate
the accuracy of error correcting operation.

[0068]As described above, there is only a part between signals (a part
where bit value is changed from 0 to 1 or from 1 to 0 between two
signals) corresponding to different bits in which bit error may be caused
and information of certainty by the LLR is required. In the outside
thereof, it is possible that the value of LLR is fixed to the maximum
value or the minimum value. Further, it is only a part between signals
crossing a decision threshold of 0 and 1 where it is necessary to change
the value of LLR according to the position of reception signal point.

[0069]Next, the assumption (b) will be described. Mapping realizing the
conditions of assumption (b) is mapping performed by gray-encoding, which
is generally adopted for making the bit error rate optimum when the error
correction code, which is the subject of the exemplary embodiment, is
used. For example, Document A disclosing AHA4541 which is an LSI for
turbo product code made by AHA Corp. also describes mapping of gray
encoding.

[0070]FIG. 5 shows gray encoding mapping of 16 QAM. If the mapping is
gray-encoded, the Pch bit is common in the symbols aligned vertically (Q
axial direction). Accordingly, it is only necessary to replace the
numerator within "exp" of the equation (2) with values of the power of
two of the distances (A, B) with the symbols corresponding to the bits 0
and 1 which are closest to the reception signal points. In this case,
although there is generally a shift C in the Q axial direction, as the
shift C is common to both of the two distances, it is eliminated when
being entered into the LLR equation (1). As shown in FIG. 6, assuming
that the orthogonal projection of a reception signal point to a
horizontal line linking two signal points is a point R and distances to
the signal points 0 and 1 are Ap and Bp respectively, the numerator
within "exp" in the equation (2) entered into the equation (1) is shown
as an equation (4).

A2-B2=(Ap2+C2)-(Bp2+C2)=Ap2-Bp2 equation (4)

[0071]As shown in the equation (4), an LLR can finally be calculated only
from the orthogonal projection (that is, one channel value) of the
reception signal point. Of course, this is the same for Qch. In this way,
even though the LLR calculation is a two-dimensional modulation method,
if it can be dissolved into two independent one-dimensional signals, the
modulation method can be considered one-dimensionally. As such, it is
understood that an LLR can be easily calculated from the coordinates of
the reception signal point.

[0072]In the case of multilevel QAM, signal point distributions, in which
0 and 1 are allocated, differ due to bit levels (MSB, 2SB, LSB).
Therefore, although it is necessary to change the calculation circuit due
to the level of bits for which the LLR is to be calculated, if it is
considered within the area where the LLR value varies, the level of the
bit has no relation. As such, the circuit of the part where the LLR
varies corresponding to the coordinates of the reception signal point is
common. In other words, it is only necessary to change the output of the
common LLR calculation circuit according to the level of the bit.

[0073]As described above, the two assumptions (a) and (b) provide an
effect of significantly simplifying the calculation procedures of LLR,
without inversely affecting the arithmetic accuracy of the LLR and
freedom in realizing apparatuses substantially. Further, as there is no
need to consider effects of a signal point which is not the subject, the
relative positional relationship between the reception signal point and
the signal point and the relation with LLR values do not depend on bit
levels (MSB, 2SB, LSB).

[0074]FIG. 4 shows a state of LLR values in Pch of 16 QAM. In MSB, there
is an inclination in one place (between P1 and P2). Further, in LSB,
there are two inclinations (between P0 and P1, and between P2 and P3),
and their polarities are opposite. The levels of inclinations are same in
MSB and LSB. This is also the case of Qch.

[0075]In FIG. 4, an LLR corresponding to the bit "0" is set to be a
maximum value, and an LLR corresponding to the bit "1" is set to be a
minimum value. This is because when the part of the soft-decision bit
indicating the position of the reception signal point is seen as an
expression of the complement of 2, the MSB coincides with the
hard-decision value of the bit. Further, the reason of seeing as the
expression of the complement of 2 is because it is convenient when
performing numeral value operation based on the LLR by a decoder, so it
is not essential. Further, the polarity of the LLR can be set while
considering consistency with the surrounding circuit.

[0076]As LLR values in the area where the LLR changes corresponding to the
position of the reception signal point become the same regardless of the
values of the hard-decision bits, the LLR can be calculated while only
referring to the soft-decision bit. Therefore, even if the number of
soft-decision bits is 5 for example, there are 32 ways of LLRs at most.
Even if this part is created using an ROM, the circuit size can be
extremely small. Further, if it is acceptable that the position between
signal points and the LLR are in proportional relationship, the
soft-decision signal of the reception signal can be the LLR directly, so
an LLR calculation circuit is not required. That is, the LLR circuit 102
shown in FIG. 1 can be created by only using the connection. In general,
sufficient characteristics can be obtained by this method. If it is
desired to change the gain of the LLR, it is only necessary to change the
inclination of the LLR by using a multiplexer inside the decoder which
actually operates the LLR, or the like.

[0077]To fix the value of the LLR according to the position of the
reception signal point and to invert the LLR, an output signal of the LLR
calculation circuit 102, it is only necessary to input a signal generated
by inverting the output signal, and input fixed values (minimum value and
maximum value), to the selection circuit 110. Then, according to the
output of the area detection circuit, the selection circuit 110 selects
and outputs any one of the inputs.

[0078]Further, as Pch and Qch are same in the mapping of gray encoding,
the same two circuits can be applied to Pch and Qch channels,
respectively.

[0079]In view of the above, the LLR arithmetic circuit of 16 QAM can be
configured as shown in FIGS. 1 and 2.

[0080]Next, specific operation of the LLR arithmetic circuit will be
described. FIG. 7 is a flowchart showing an exemplary process of
calculating an LLR by the LLR arithmetic circuit. Although the case of
calculating an LLR on the Pch side will be described below, the case of
calculating an LLR on the Qch side is also the same. This means, in the
below description, calculating an LLR on the Qch side can be described by
replacing the area detection circuit 101A with the area detection circuit
101B, replacing the LLR circuit 102A with the LLR circuit 102B, replacing
the LLR converters 103A and 104A with the LLR converters 103B and 104B,
and replacing the P axis with the Q axis.

[0081]In the process of calculating an LLR, first, to the area detection
circuit 101A of the LLR arithmetic circuit, a hard-decision bit among
bits indicating P-axis coordinates of the reception signal point is
input. Then, based on the hard-decision bit input, the area detection
circuit 101A detects an area on the phase plane where the P-axis
coordinates of the reception signal point are present (step S11). Then,
the area detection circuit 101A outputs the detection result of the area
on the phase plane to the LLR converters 103A and 104A.

[0082]In the step S11, the area detection circuit 101A specifically
detects that whether the area on the phase plane where P-axis coordinates
of the reception signal point are present is (1) an area in which an LLR
changes with a positive inclination crossing the hard-decision threshold,
(2) an area where an LLR changes with a negative inclination crossing the
hard-decision threshold, (3) an area where an LLR does not cross the
hard-decision threshold and does not change at the maximum value, or (4)
an area where an LLR does not cross the hard-decision threshold and does
not change at the minimum value, and then the area detection circuit 101A
outputs the result.

[0083]Further, to the LLR circuit 102A, a soft-decision bit among the bits
indicating P-axis coordinates of the reception signal point is input.
Then, based on the soft-decision bit input, the LLR circuit 102A
calculates the primary LLR (step S12). Then, the LLR circuit 102A outputs
the calculated primary LLR to the LLR converters 103A and 104A.

[0084]To the selection circuits 110 of the LLR converters 103A and 104A,
the output signal (primary LLR) of the LLR circuit 102A, the output
signal of the LLR circuit 102A which is inverted by the inverter 111, the
predetermined LLR maximum value, and the predetermined LLR minimum value
are input. Then, based on the detection result of the area detection
circuit 101A, the selection circuit 110 selects one of the output signal
of the LLR circuit 102A, the output signal inverted by the inverter 111,
the predetermined LLR maximum value, and the predetermined LLR minimum
value (step S13). Then, the selection circuit 110 outputs the selection
result of the step S13 as the final LLR (step S14).

[0085]Specifically, if the detection result of the area detection circuit
101A is the above item (1), that is, an area in which the LLR changes
with positive inclination crossing the hard-decision threshold, the
selection circuit 110 directly selects and outputs the output signal of
the LLR circuit 102. If the detection result of the area detection
circuit 101A is the above item (2), that is, an area in which the LLR
changes with a negative inclination crossing the hard-decision threshold,
the selection circuit 110 selects and outputs the output signal of the
LLR circuit 102A which is inverted by the inverter 111. If the detection
result of the area detection circuit 101A is the above item (3), that is,
an area in which the LLR does not cross the hard-decision threshold and
does not change at the maximum value, the selection circuit 110 selects
and outputs the predetermined LLR maximum value. If the detection result
of the area detection circuit 101A is the above item (4), that is, an
area in which the LLR does not cross the hard-decision threshold and does
not change at the minimum value, the selection circuit 110 selects and
outputs the predetermined LLR minimum value.

[0086]Through the process described above, the selection circuit 110
calculates the soft-decision signal shown one-dimensionally between the
adjacent signal points over the two (0, 1) decision thresholds as a log
likelihood ratio directly. Further, the selection circuit 110 calculates
the soft-decision signal shown one-dimensionally between the adjacent
signal points not crossing the two (0, 1) decision threshold as a log
likelihood ratio while fixing to the predetermined maximum value or to
the predetermined minimum value, according to the position of the signal
point on the phase plane.

[0087]Next, the differences between the apparatus described in Patent
Document 1 and the LLR arithmetic circuit shown in the exemplary
embodiment will be described. In Patent Document, LLR calculation is
performed by using only the above-described assumption (b). Although it
is common to the exemplary embodiment in using a concept that the
gray-encoded mapping can be considered one-dimensionally, as the
assumption (a) is not taken into account, the apparatus in Patent
Document 1 calculates the LLR using the equation (4). Therefore, the
amount of operation for calculating the LLR cannot be reduced
sufficiently. In the exemplary embodiment, the amount of operation is
reduced more than that of the apparatus described in Patent Document 1
without deteriorating the characteristics substantially, due to the
effect of the assumption (a).

[0088]The arithmetic method given above will be described below by
exemplary using the case of 16 QAM. It is assumed that the coordinates of
the reception signal are expressed as natural codes in which the
hard-decision bit and the lower soft-decision bit are combined (in Pch,
the left ends are all "0" and the right ends are all "1").

[0089]First, an LLR with respect to the hard-decision MSB of Pch will be
described. MSB of Pch is formed such that 8 pieces in the right half of
the signal point are 1, and 8 pieces in the left half are 0 (refer to
FIG. 5) Therefore, the part which forms a decision threshold of the
values of MSB of Pch is the Q axis.

[0090]If the reception signal point is present in the left from the signal
point line on the immediately left side of the Q axis, there is no
possibility that the transmission signal is "1", so the LLR is the
maximum value. In contrast, if the reception signal point is present in
the right from the signal point line on the immediately right side of the
Q axis, there is no possibility that the transmission signal is "0", so
the LLR is the minimum value. As an error may be caused in Pch MSB only
when the reception signal point is in between signal points over the Q
axis, the LLR value corresponding to the position of the reception signal
point is needed. As the probability is low (probabilities of 0 and 1 are
near) when the reception signal point is near the Q axis, the absolute
value of LLR becomes smaller. As the reception signal point is away from
the Q axis, the probability that the transmission signal is "0" or "1"
becomes higher, so the absolute value of LLR becomes larger.

[0091]If the soft-decision bit is the LLR, it is the complement of 2, and
it is a negative value when it is larger than the threshold, while it is
a positive value when it is smaller than the threshold. This MSB
coincides with the hard-decision value of the bit. FIG. 6 shows the case
where the soft-decision bits are 3 bits.

[0092]FIG. 8 illustrates an example showing LLR area sections with respect
to the MSB of Pch. As shown in FIG. 8, there are three areas in the LLR
area sections to the MSB of Pch, including, from the left side, an area
501 where the LLR is the maximum value, an area 502 where the LLR changes
from positive and negative, and an area 503 where the LLR is the minimum
value.

[0093]Further, the LLR with respect to the MSB of Qch is also determined
in the same manner as that of MSB of Pch.

[0094]Next, an LLR with respect to 2SB (2nd Significant Bit, in this
case, also LSB) of Pch will be described. The 2SB of Pch includes 0, 1,
1, and 0 from the signal point line on the left, and there are two parts
forming decision thresholds. At these two positions (between signal
points), the LLR changes as the case of MSB. In this case, although the
bits are in the order of 0 and 1 at the thresholds positions on the left,
which is the same as that of MSB, the bits are in the order of 1 and 0 at
the thresholds positions on the right, which is in the opposite order to
that of MSB. Therefore, it is necessary to change the polarity of the LLR
at the two thresholds positions.

[0095]Since no error will be caused between signal points where both of
the bits are 1, the LLR is set to be the minimum value (negative maximum
absolute value). Further, since no error will be caused on the outside
from the two lines on the both sides on the phase plane, the LLR is set
to be the maximum value (positive maximum absolute value) (refer to FIG.
4B).

[0096]FIG. 9 illustrates an example showing LLR area sections with respect
to the 2SB (LSB) of Pch. As shown in FIG. 9, the LLR area sections with
respect to 2SB (LSB) of Pch have five areas, including, from the left, an
area 551 where the LLR is the maximum value, an area 552 where the LLR is
changed from positive to negative, an area 553 where the LLR is the
minimum value, an area 554 where the LLR is changed from negative to
positive, and an area 555 where the LLR is the maximum value.

[0097]The LLR with respect to 2SB of Qch is set in the same manner as that
of 2SB of Pch.

[0098]Further, in the case of applying to a modulation method in which
multilevel signal points are the even power of two (e.g., 64 QAM, 256
QAM, etc), it is only necessary to add 3SB or after in the similar
manner.

[0099]In the circuit configuration of the LLR arithmetic circuit shown in
the exemplary embodiment, the circuit size will not be changed
significantly even if the modulation multilevel number increases.
Therefore, as the multilevel number is larger, the reduction rate of the
circuit size becomes larger compared with the circuit configuration for
realizing a circuit for LLR operation using a ROM. As such, an LLR
arithmetic circuit can be realized without a need of a large-scale ROM,
regardless of the modulation method. Accordingly, it is easy to realize
high-speed operation, compared with the related circuit configuration for
LLR arithmetical operation.

[0100]As described above, according to the exemplary embodiment, it is
possible to dissolve the LLR calculation which is originally performed
two-dimensionally into one-dimensionally and to significantly reduce the
amount of operation, in the communication system using QAM. Further, as
all or the almost all of the LLR arithmetic operation can be performed by
a logical arithmetic unit, there is no need to used a large-scale ROM,
and the circuit size and the power consumption can be reduced regardless
of the modulation method and the bit accuracy of LLR. Further, the
operating speed of the circuit can be improved, and consequently, a
large-capacity communication system can be realized. Accordingly, it is
possible to realize higher-speed LLR calculation while reducing the
circuit size.

[0101]Although the elements (101A, 101B, 102A, 102B, 103A, 103B, 104A,
104B) configuring the LLR arithmetic circuit shown in FIGS. 1 and 2 are
constructed as hardware, it is also acceptable to construct the functions
performed by those elements as programs which are software, and to cause
a computer to perform those programs to thereby carry out the processes
performed by the LLR arithmetic circuit.

Second Exemplary Embodiment

[0102]Next, a second exemplary embodiment of the present invention will be
described with reference to the drawings. Although the first exemplary
embodiment has described the case where the number of symbols is the even
power of two, the exemplary embodiment describes the case where the
number of symbols is the odd power of two. Before the specific
configuration and operation of the LLR arithmetic circuit is described, a
method of calculating an LLR when the number of symbols is the odd power
of two will be described first. Gray-encoded mapping can be realized only
when the case of using a quadrature modulation method in which the number
of signal points is even power of two. In the case of using a modulation
method in which the number of signal points is the odd power of two
(e.g., 32 QAM, 128 QAM), gray encoding cannot be realized.

[0103]However, it is possible to create a map in which parts where a bit
difference (humming distance) between adjacent signal points is 2 or more
is made as few as possible by using the concept of gray-encoding.
Hereinafter, this is called quasi-gray encoding in the exemplary
embodiment.

[0104]FIG. 10 is a map showing quasi-gray encoding in the case of using 32
QAM. In the quasi-gray encoding, MSB (highest bit) of the orthogonal two
channels is gray-encoded. Further, LSB (lowest bit) may also be
gray-encoded depending on the multilevel number. Accordingly, these bits
can be considered by dissolving into one dimension in the same manner as
that of the modulation method where the number of signal points is the
even power of two.

[0105]Although the bits which are not gray-encoded are arranged
two-dimensionally, when the mapping is determined using the concept of
gray encoding, the number of threshold values of 0, 1 decision can be
reduced by treating them as the same group of bit. FIG. 10 shows an
exemplary map of 32 QAM calculated based on that concept. In FIG. 10,
each of the numbers surrounded by a square is MSB. Further, those
positioned near the symbol are 3 bits other than MSB.

[0106]FIG. 11 illustrates an example of LLR area for 3SB (left end of 3
bits) of 32 QAM. As shown in FIG. 11, 3SB of 32 QAM is not divided in one
direction which is different from the case of each area being
gray-encoded, but is divided in two directions of P axis and Q axis. In
FIG. 11, regarding a part 605 where the areas divided in two directions
overlap, it is necessary to calculate the LLR while considering
information of the two directions. Hereinafter, such areas are called
special areas. As shown in FIG. 11, in 3SB of 32 QAM, four special areas
are present on the phase plane, each of which is placed in each quadrant.
There are some types of special areas depending on the relationship with
signal points or bits, and the way of processing LLR calculation differs
corresponding to the type of the special area.

[0107]FIG. 12 illustrates examples of special areas. In FIG. 12, two types
of special areas are exemplary shown. FIG. 12A shows the case where
signal points are present in four quadrants of the special area, and
three bits among the four bits of the signal point are the same. As the
special area is divided into four areas by a bit which is one-level lower
than the hard-decision bit, the numbers same as those of the quadrants of
the orthogonal coordinates are allocated to the four areas (see FIG.
12C).

[0108]First, the case where the reception signal point is present in the
quadrant 1 will be considered. In the quadrant 1 and the quadrant 2, the
bits are 1, that is, the same, and no error will be caused. However, as
the bits are different in the quadrant 1 and the quadrant 4, that is, 1
and 0, an error may be caused, and the soft-decision value in Q-axial
direction becomes the LLR. In the case where the reception signal point
is present in the quadrant 2, as the bit is the same as those of the
quadrant 1 and the quadrant 3, that is, 1, no error will be caused, so
the LLR is the minimum value. In the case where the reception signal
point is present in the quadrant 3, the bit is different from that of the
quadrant 4, that is, 1 and 0, so the soft-decision value in the P-axial
direction becomes the LLR.

[0109]Further, in the case where the reception signal point is present in
quadrant 4, as the bits are 1 and 0 with the quadrants 1 and 3, two LLRs
will be obtained from the soft-decision values in the P-axial direction
and the Q-axial directions. Then, one having the smaller absolute value
(having higher possibility of error) of the two LLRs is selected as the
LLR.

[0110]FIG. 12B shows the case where signal points are present at only
three corners of the special area. The way of dividing the special area
is the same as that shown in FIG. 12A. As two directions can be
considered when the reception signal point is present in the quadrant 1
and when the signal point is present in the quadrant 3, one having the
smaller absolute value of the two LLRs is selected. Further, as there is
no effect from absent signal points, when the reception signal point is
present in the quadrant 2 and when the reception signal point is present
in the quadrant 4, the LLR is obtained for one direction where the signal
point is present.

[0111]In summary, even in any special area, it is possible to define
signal processing in the four or three quadrants within the area
according to the rules described below.

(Rule 1): If bits in two directions (P-axial direction and Q-axial
direction) are the same, the LLR is the maximum value or the minimum
value.(Rule 2): If a bit in only one direction (P-axial direction or
Q-axial direction) is different, the LLR in a direction where the bit is
different is selected.(Rule 3): If both bits in two directions (P-axial
direction and Q-axial direction) are different, one having a smaller
absolute value of the two LLRs is selected as the LLR.(Rule 4): For a
direction with no signal point, no bit error will be caused.(Rule 5): If
the reception signal point is present in an area with no signal point, an
LLR having a smaller absolute value, among LLRs in P-axial direction and
Q-axial direction, is selected.

[0112]For 4SB and 5SB (LSB), an LLR in a special area can be obtained by
applying the same concept although the sections of areas are different.
As such, LLRs of all bits can be calculated by the arithmetic circuit,
together with MSB (of two channels) in which gray encoding is possible.

[0113]As described above, as a special area is present in the case of 32
QAM so that it is necessary to perform area decision using a lower bit of
the hard-decision bit, the LLR arithmetic circuit in the case of using 32
QAM can be configured as shown in FIGS. 13 and 14.

[0114]FIG. 13 is a block diagram showing another exemplary configuration
of an LLR arithmetic circuit, and FIG. 14 is a block diagram showing
another exemplary configuration of the LLR converters 104C, 104D and
104E. As shown in FIG. 14, in the exemplary embodiment, an LLR arithmetic
circuit includes, a processing circuit for a normal area which calculates
LLRs for a normal area including only bits which are completely
gray-encoded among the quasi-gray encoded bits output from an encoder,
and a processing circuit for a special area which calculates LLRs for a
special area including bits which are not completely gray-encoded among
the quasi-gray encoded bits output from an encoder.

[0115]Hereinafter, operation of the LLR arithmetic circuit shown in FIGS.
13 and 14 will be described. First, as it is expressed by gray codes in
MSB (P/Q 2 bits), operation of the LLR arithmetic circuit is the same as
that of 16 QAM.

[0116]Next, in the case of 3-5SB (3 bits), when processing of a normal
area is to be performed, the LLR arithmetic circuit may (1) output the
maximum value (fixed value) of the LLR, (2) output the minimum value
(fixed value) of the LLR, (3) directly output the LLR if the inclination
of the LLR is positive, or (4) generate and output a signal in which all
bits of the LLR are inverted if the inclination of the LLR is negative.
In this case, the LLR includes P direction and Q direction. Based on the
area decision result (2 bits) of the area detection circuit 201, the LLR
arithmetic circuit selects an output of any one of the items (1) to (4)
above.

[0117]In this case, an area decision result (REG_N) indicates precision of
specifying one area surrounded by four adjacent signal points. Further,
an LLR is information indicating only the soft-decision part among bits
indicating the reception signal point.

[0118]In the case of performing a process of a special area, the LLR
arithmetic circuit may (1) output the maximum value (may be positive or
negative, the polarity is determined by the bit string (3-5SB)) of the
absolute value, (2) always output an LLR in P direction, (3) always
output an LLR in Q direction, or (4) output an LLR having the smaller
absolute value (min (|P|, |Q|) of LLRs in P direction and Q direction,
using an LLR absolute value comparator. In this case, the LLR arithmetic
circuit selects any one of the above items (1) to (4) based on the
quadrant decision result (REG_S) inside the special area by the area
detection circuit 201. Note that the selected signals are 4 bits as there
are four quadrants in each of the four special areas. Further, the
polarity may be opposite in the outputs (2) to (4).

[0119]As special areas are different in the respective bit strings 3-5SB,
an output of area decision with respect to the same reception signal
point is different for each bit string.

[0120]Further, the LLR arithmetic circuit selects and outputs any one of
the two results (output of a processing circuit in the normal area and
output of a processing circuit in the special area) depending on the
separating signal of the normal area and the special area. In this case,
for an output not selected by the LLR arithmetic circuit, any signal may
be output.

[0121]The area decision circuit (area detection circuit 201) includes two
types of circuits, that is, DET 1 for bit strings in a Gray Mapping, and
DET 2 for bit strings which are not in a Gray Mapping. Further, the DET 2
outputs three type of signals, including REG_N for the normal area, REG_S
for the special area, and REG_N for separating the normal area and the
special area.

[0122]Note that the circuit design shown in the exemplary embodiment is an
example, and in the actual circuit design, there are a plurality of ways
of realizing LLR arithmetic circuits other than the LLR arithmetic
circuit shown in the exemplary embodiment. In the exemplary embodiment,
the normal area and the special area are described while being separated
such that the necessary signal processing is easily understood.

[0123]In the case of applying to a modulation method in which the
multilevel signal points are the odd power of two (e.g., 128 QAM, 512
QAM), it is only necessary to set an LLR area of bit strings which cannot
be gray-encoded in the same way of thinking as described above. In a
modulation method for 128 QAM or more (e.g., 512 QAM), LSB (2 bits) can
also be gray-encoded, which can be realized with a circuit simpler than
that of 32 QAM.

[0124]In the circuit configuration of the LLR arithmetic circuit shown in
the exemplary embodiment, the circuit size will not change significantly
even if the modulation multilevel number becomes larger. Therefore, as
the multilevel number is larger, the circuit size reduction rate becomes
larger compared with the circuit configuration in the case of realizing a
circuit for LLR operation using a ROM. Therefore, it is possible to
realize an LLR arithmetic circuit without a need of a large-scale ROM,
regardless of the modulation method. As such, higher-speed operation can
be realized easily compared with a related circuit configuration for LLR
operation.

[0125]Although the elements (102A, 102B, 201, 103A, 103B, 104C, 104D,
104E, 112, 113 and 114) configuring the LLR arithmetic circuit shown in
FIGS. 13 and 14 are constructed as hardware, the functions performed by
these elements may be constructed as programs which are software to be
executed by a computer such that processing performed by the LLR
arithmetic circuit is carried out.

[0126]As described above, according to the exemplary embodiment, even if
QAM having a signal point of the odd power of two is used, it is possible
to perform LLR calculation at a higher speed while reducing the circuit
size by performing quasi-gray encoding.

[0127]Note that the LLR arithmetic circuit shown in each of the exemplary
embodiments can be applied to error correction codes for iterative
decoding of a soft-decision signal and a decoder therefor. Examples of
those codes which have been known include turbo (convolution) code, TPC
(turbo product code), and LDPC (Low-Density Parity-Check).

[0128]Further, with a used of the LLR arithmetic circuit shown in each of
the exemplary embodiments, an LLR can be used as a metric of an input of
an equalizer. As such, the LLR arithmetic circuit shown in each of the
exemplary embodiments can be used as an LLR arithmetic circuit of an
input signal of an equalizer by maximum likelihood sequence estimation,
besides an error correction code decoder.

[0129]Further, in each of the exemplary embodiments, the LLR arithmetic
circuit may include an ROM to which a soft-decision signal shown
one-dimensionally between adjacent signal points having two (0, 1)
decision thresholds between them is input and which outputs a log
likelihood ratio calculated beforehand, or a log likelihood ratio output
circuit including a logic circuit corresponding thereto. Further, it is
also acceptable that a log likelihood ratio output circuit is shared by
all reception signal points, and according to the position on the phase
plane of the reception signal points, an output of the LLR may be output
by being replaced with a fixed value.

[0130]Next, another exemplary embodiment of the invention will be
described.

[0131]A log likelihood ratio arithmetic circuit may set the correspondence
relationship between bit and symbol such that the bit error rate becomes
the minimum. Further, a log likelihood ratio arithmetic circuit may set
the correspondence relationship between bit and symbol, when the
modulation method has signal points of the even power of two, to be in a
Gray mapping. Further, a log likelihood ratio arithmetic circuit may set
the correspondence relationship between bit and symbol, when the
modulation method has signal points of the odd power of two, to be in a
quasi-Gray mapping where the bit error rate becomes the minimum. Further,
it is also acceptable that a log likelihood ratio arithmetic circuit may
perform all or almost all operation for calculating a log likelihood
ratio by logic operation using information of reception signal point
coordinates.

[0132]Further, a log likelihood ratio arithmetic circuit may output the
part of soft-decision bits of 1CH among information of reception signal
point coordinates indicated in 2CH directly or by inverting them, as a
log likelihood ratio of an area where the log likelihood ratio varies.
Further, a log likelihood ratio arithmetic circuit may output the maximum
value or the minimum value of the log likelihood ratio as the log
likelihood ratio of an area where the log likelihood ratio does not vary.
Further, if there is an area which cannot be determined based on only
information of hard-decision bit of 1CH among information of reception
signal point coordinates shown in 2CH, it is also acceptable that for an
area where a bit error will never be caused under the conditions
described in claim 1, a log likelihood ratio arithmetic circuit outputs
the maximum value or the minimum value of the log likelihood ratio, and
for an area where a bit error may be caused, the log likelihood ratio
arithmetic circuit outputs a soft-decision bit of lower likelihood of
2CH.

[0133]Further, in a log likelihood ratio arithmetic circuit, selection of
a log likelihood ratio may be performed by using a result of area
decision based on information of hard-decision bit in 1CH of information
of reception signal coordinates indicated in 2CH. Further, in a log
likelihood ratio arithmetic circuit, selection of a log likelihood ratio
may be performed by using a result of area decision by referring to bits
one level lower than the hard decision, if there is an area where area
decision cannot be performed only based on information of hard-decision
bit in 1CH of information of reception signal point coordinates indicated
in 2CH. Further, in a log likelihood ratio arithmetic circuit, a log
likelihood ratio may be one which is converted from a soft-decision bit
to another value.

[0134]The log likelihood ratio (LLR) arithmetic circuit according to the
exemplary embodiment does not refer to a predetermined table but directly
calculates an LLR from reception signal point coordinates through
arithmetic operation using actual time. With such a configuration,
compared with the case of configuring a circuit using a ROM, the circuit
size can be extremely small, and also high-speed operation can be
realized. Further, the log likelihood ratio arithmetic circuit according
to the present invention dissolves reception signal point coordinates in
two dimensions into two one-dimensional signals, and performs LLR
operation for each bit allocated to a symbol. As such, a log likelihood
ratio arithmetic circuit can also be realized using a small circuit with
a little delay such as a selection circuit, a large and small comparator
or an inverter. Accordingly, the circuit configuration is simplified by
dissolving the coordinates of a reception signal point which is in two
dimensions originally into two one-dimensional signals. Further, a
modulation method applicable to the log likelihood ratio arithmetic
circuit according to the exemplary embodiment has no restricting
conditions in the number of symbols. Further, as the LLR itself is
simplified in the scope of not affecting the error correction decoding
characteristics, no square distance operation is required.

[0135]Although the present invention has been described by referring to
the exemplary embodiments, the present invention is not limited to the
above-described exemplary embodiments. The configuration and the details
of the present invention can be modified within the scope of the present
invention in various ways which can be understood by those skilled in the
art.

[0136]The present application claims priority based on Japanese Patent
Application No. 2006-266523 filed on Sep. 29, 2006, the entire disclosure
of which is incorporated herein.

INDUSTRIAL APPLICABILITY

[0137]The present invention is applied to communication systems using
quadrature multilevel modulation method (quadrature amplitude modulation
method), and is applicable to an LLR calculation circuit which calculates
a log likelihood ratio serving as an input signal of an error correction
decoder and an equalizer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0138]FIG. 1 is a block diagram showing an exemplary configuration of a
log likelihood ratio arithmetic circuit (LLR arithmetic circuit)
according to the present invention.

[0139]FIG. 2 is a block diagram showing an exemplary circuit configuration
of an LLR converter.

[0140]FIG. 3 is an illustration showing a positional relationship between
an LLR and a reception signal point in BPSK.

[0141]FIG. 4 is an illustration showing an example of a positional
relationship between an LLR and a reception signal point in 16 QAM.